# filterMap
> Filter and map elements in an input [ndarray][@stdlib/ndarray/ctor] to elements in a new output [ndarray][@stdlib/ndarray/ctor] according to a callback function.
## Usage
```javascript
var filterMap = require( '@stdlib/ndarray/filter-map' );
```
#### filterMap( x\[, options], fcn\[, thisArg] )
Filters and maps elements in an input [ndarray][@stdlib/ndarray/ctor] to elements in a new output [ndarray][@stdlib/ndarray/ctor] according to a callback function.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var ndarray = require( '@stdlib/ndarray/ctor' );
function fcn( z ) {
if ( z > 5.0 ) {
return z * 10.0;
}
}
var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;
var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns
var y = filterMap( x, fcn );
// returns [ 80.0, 90.0, 100.0 ]
```
The function accepts the following arguments:
- **x**: input [ndarray][@stdlib/ndarray/ctor].
- **options**: function options _(optional)_.
- **fcn**: callback function.
- **thisArg**: callback function execution context _(optional)_.
The function accepts the following options:
- **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. If not specified, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor].
- **order**: index iteration order. By default, the function iterates over elements according to the [layout order][@stdlib/ndarray/orders] of the provided [ndarray][@stdlib/ndarray/ctor]. Accordingly, for row-major input [ndarrays][@stdlib/ndarray/ctor], the last dimension indices increment fastest. For column-major input [ndarrays][@stdlib/ndarray/ctor], the first dimension indices increment fastest. To override the inferred order and ensure that indices increment in a specific manner, regardless of the input [ndarray][@stdlib/ndarray/ctor]'s layout order, explicitly set the iteration order. Note, however, that iterating according to an order which does not match that of the input [ndarray][@stdlib/ndarray/ctor] may, in some circumstances, result in performance degradation due to cache misses. Must be either `'row-major'` or `'column-major'`.
By default, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor]. To return an ndarray with a different [data type][@stdlib/ndarray/dtypes], specify the `dtype` option.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var ndarray = require( '@stdlib/ndarray/ctor' );
var dtype = require( '@stdlib/ndarray/dtype' );
function fcn( z ) {
if ( z > 5.0 ) {
return z * 10.0;
}
}
var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;
var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns
var opts = {
'dtype': 'float32'
};
var y = filterMap( x, opts, fcn );
// returns [ 80.0, 90.0, 100.0 ]
var dt = dtype( y );
// returns 'float32'
```
To set the callback function execution context, provide a `thisArg`.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var ndarray = require( '@stdlib/ndarray/ctor' );
function fcn( z ) {
this.count += 1;
if ( z > 5.0 ) {
return z * 10.0;
}
}
var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;
var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns
var ctx = {
'count': 0
};
var y = filterMap( x, fcn, ctx );
// returns [ 80.0, 90.0, 100.0 ]
var count = ctx.count;
// returns 6
```
The callback function is provided the following arguments:
- **value**: current array element.
- **indices**: current array element indices.
- **arr**: the input [ndarray][@stdlib/ndarray/ctor].
## Notes
- The function does **not** perform explicit casting (e.g., from a real-valued floating-point number to a complex floating-point number). Any such casting should be performed by a provided callback function.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var ndarray = require( '@stdlib/ndarray/ctor' );
var Complex128 = require( '@stdlib/complex/float64/ctor' );
function fcn( z ) {
if ( z > 5.0 ) {
return new Complex128( z, 0.0 );
}
}
var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;
var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns
var opts = {
'dtype': 'complex128'
};
var y = filterMap( x, opts, fcn );
// returns [ [ 8.0, 0.0 ], [ 9.0, 0.0 ], [ 10.0, 0.0 ] ]
```
- If a provided callback function returns `undefined`, the function skips the respective [ndarray][@stdlib/ndarray/ctor] element. If the callback function returns a value other than `undefined`, the function stores the callback's return value in the output [ndarray][@stdlib/ndarray/ctor].
- The function **always** returns a one-dimensional [ndarray][@stdlib/ndarray/ctor].
## Examples
```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var array = require( '@stdlib/ndarray/array' );
var filterMap = require( '@stdlib/ndarray/filter-map' );
function fcn( v ) {
if ( v > 0 ) {
return v * 100;
}
}
var buffer = discreteUniform( 10, -100, 100, {
'dtype': 'generic'
});
var x = array( buffer, {
'shape': [ 5, 2 ],
'dtype': 'generic'
});
console.log( ndarray2array( x ) );
var y = filterMap( x, fcn );
console.log( ndarray2array( y ) );
```
[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray/tree/main/ctor
[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/ndarray/tree/main/dtypes
[@stdlib/ndarray/orders]: https://github.com/stdlib-js/ndarray/tree/main/orders
[@stdlib/ndarray/filter]: https://github.com/stdlib-js/ndarray/tree/main/filter
[@stdlib/ndarray/map]: https://github.com/stdlib-js/ndarray/tree/main/map
[@stdlib/ndarray/reject]: https://github.com/stdlib-js/ndarray/tree/main/reject
[@stdlib/ndarray/slice]: https://github.com/stdlib-js/ndarray/tree/main/slice